"Now Maliki and the American are turning their sights on the other major armed group outside the government," he said.

"There will be a lot of violence if Maliki attempts to eliminate the al-Mahdi army.

"The Sadr movement is a very large social movement, the Mahdi army is to some extent street gangs and young men with guns and you can't crush a thing like that very easily, its organic.

"You can't take a social movement out and shoot it."

Clashes between fighters and US and Iraqi security forces in the predominantly Shia neighbourhood of Sadr City have killed more than 900 people, according to Iraqi officials.

"There were 925 martyrs in Sadr City and 2,605 others have been wounded," Tehseen Sheikhly, spokesman for the government's Baghdad security plan, said.

Militia crackdown

The fighting in the eastern neighbourhood of the capital began after al-Maliki targeted militias in the southern city of Basra. The crackdown triggered a wave of fighting across Shia areas.

Sheikhly insisted that the offensive in Sadr City "was not targeting any political party or group but armed groups".

"There will be a lot of violence if al-Maliki attempts to eliminate the al-Mahdi army"

Juan Cole, Iraq expert

Wednesday's clashes started when a US patrol came under mortar fire, according to the US military.

"The US soldiers returned fire and in the ensuing engagement killed seven militants," Lieutenant-Colonel Steven Stover, spokesman for the US army, said.

Later on Wednesday, another US patrol came under attack which damaged a military vehicle, while six other fighters died in battles with US forces, he said.

Some of the heaviest fighting has taken place in the past three days, with fighters taking advantage of blinding dust storms that ground US attack helicopters to launch ambushes on U.S. and Iraqi positions.

US forces have responded with tank fire and surface-to-surface missiles, destroying buildings.

At least 34 bodies and 112 wounded victims were brought to the two hospitals in Sadr City in the last 24 hours, hospital officials told the Reuters news agency.